1. Field of the Invention
The present invention relates to monostable or "one shot" multivibrators and is particularly directed to monostable multivibrators capable of generating output pulses of varying pulse widths in response to variation in the pulse repetition rate of the input triggering pulses.
It is desirable in a number of applications in computer technology and particularly in high speed thermal printing to accurately generate pulse trains of varying pulse widths consistently in response to rapidly changing input conditions. In thermal printing, a matrix of heating elements are energized in response to a plurality of electrical signals to insure that the printing faces of the heating elements are energized and de-energized in the proper sequence and for the correct time durations. A typical thermal printer of the "on-the-fly" type is described by U.S. Pat. No. 3,777,116 in which the thermally sensitive paper is continuously moved during the printing operation.
The use of a monostable multivibrator to provide variable pulse width signals as the time controlled printing head drive in a thermal printer necessitates rate compenstation of the multivibrator to control its switching time, which is determinative of the ON and OFF time of the individual printing matrix elements, as the ON time of such elements must be decreased as the printing rate increases, thereby reducing the energy input per print cycle, and vice versa. It is important that the relationship between multivibrator output pulse width (the ON time of the multivibrator) and the trigger pulse repetition rate is predictable and consistent so that compensation for heating of the matrix elements and proper adjustment for varying printing rates is achieved.
2. Description of the Prior Art
The operation and construction of monostable multivibrators having a variable output pulse width and various compensation techniques for achieving such a variable pulse width are well known in the prior art. The instant invention comprises a new and improved circuit for achieving predictable and useful self-compensation without the attendant disadvantage of the prior art.
Monostable multivibrators generate an output pulse having a predetermined width in response to an input triggering pulse, with the output pulse width being independent of the input pulse width, but dependent upon the multivibrator circuit constants, usually determined by one or more RC networks. One such multivibrator of the prior art is disclosed by U.S. Pat. No. 3,530,314.
The prior art also discloses one shot multivibrators whose output pulse width is controlled in dependence upon the repetition rate of an input triggering pulse either by variation of the resistance value of the associated RC time constant components of the circuit or by variation of the effective capacitance value of the RC time constant components of the circuit, both of which techniques involve varying the circuit time constant by changing component values. The former technique is disclosed by U.S. Pat. No. 3,304,437 and the latter technique by U.S. Pat. No. 3,517,220. In U.S. Pat. No. 3,517,220, the switching transistors of the multivibrator are coupled through a variable capacitance feedback circuit in which a diode-capacitance timing network within the feedback circuit is responsive to a control voltage which controls the amount of capacitance in the feedback circuit. The value of control voltage determines the diode conductivity which in turn determines the amount of capacitance switched into or out of the circuit. This technique, and the other techniques of the known prior art have the disadvantage of requiring component variation or coupling and de-coupling of components to effect a time constant variation, which variation in turn is used to modify the multivibrator control voltage which determines the output pulse width. Additionally, the techniques of the known prior art do not provide either as linear a relationship between the output pulse width and multivibrator control voltage or as useful an operating range of trigger pulse frequencies over which consistant operation will occur.